1. Technical Field
The present invention relates to a technique for supplying a low-ripple voltage to loads.
2. Related Art
Due to the demands for thin and light design of portable electronic apparatuses, display devices incorporated in such electronic apparatuses include electro-optical elements meeting the demands, such as liquid crystal elements. A voltage necessary for driving the electro-optical elements is higher than a voltage of 3 to 5 V for operating a logic circuit in the electronic apparatuses, and fundamentally is an AC driving voltage. Thus, both positive and negative voltages are required.
Such electronic apparatuses therefore require a power supply circuit for boosting a single DC voltage of a DC power source such as a battery to produce a positive voltage and inverting the positive voltage to a negative voltage. There are many types of power supply circuits. Among them, a typical power supply circuit used in compact, simple, low-power-consumption portable electronic apparatuses is configured such that a voltage boosted by a turn-on and turn-off (or switching) operation is held and smoothed by a capacitor (or holding element) disposed at the output stage before the voltage is supplied to a load (see, for example, Japanese Unexamined Patent Application Publications No. 2000-278938 and No. 2001-117649).
Although the capacitor is disposed at the output stage of the power supply circuit, the voltage across the capacitor is boosted by switching, and is stepped down by the power consumption of the load, thus causing a ripple. The ripple causes the root-mean-square (RMS) voltage applied to the electro-optical elements to change, and affects the display quality. Thus, the lower the ripple, the better.